The I-type of OGIs, which suffered from the process of early biodegradation and later-hydrocarbon input and featured high concentrations of 17a(H)-diahopane and app-regular steranes
, mainly originated from the shallow source rocks of the Middle Jurassic strata, while the II-type one with a series of long-chain alkylnaphthalene may originate from the lower portion of Middle Jurassic or deeper source rocks than the Middle Jurassic strata.
The biomarkers data were obtained from gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analysis of normal alkanes, isoprenoids, triterpene, and steranes
. Carbon isotopes (I'13C) include saturated and aromatic fractions.
Biomarkers such as hopanes and steranes
have received some attention, due to their usefulness as indicators of organic matter type and quality, depositional conditions (e.g.
"The combined Neoproterozoic demosponge sterane
record, showing 24-ipc and 26-mes steranes
co-occurring in ancient rocks, is unlikely attributed to an isolated branch or extinct stem-group of demosponges," Love said.
Hydrocarbons, such as n-alkanes, hopanes, and steranes
, are also important for investigating contamination by crude oil and its derivatives in recent sediments [10, 11].
Organic matter extracted from Lower Silurian Longmaxi Fm, Upper Permian Longtan Fm, and Dalong Fm source rocks shows very similar distributions of 191 and 217 traces, including abundant gammacerane, and "V" shape in the distribution of [C.sub.27]-[C.sub.29] regular steranes
Alkanes are the key components of many refined petroleum products (e.g., gasoline, diesel fuel, kerosene, jet fuel, and heating oil) and are subdivided into several classes which include the following: linear alkanes (n-alkanes), branched alkanes, isoprenoids, cycloalkanes (e.g., steranes
and triterpanes), and unresolved complex mixtures (UCM) which are difficult to resolve by capillary column gas chromatography [6, 16].
5a-b) is attributed to terrestrial organic matter (Simoneit et al, 1986), while plots of [C.sub.27] [C.sub.28] and [C.sub.29] monoaromatic steranes
The relative content of [C.sub.27]-, [C.sub.28]-, and [C.sub.29]- regular steranes
can indicate the original organic matter inputs of source rocks .
and Moldowan J.M.: "The effect of biodegradation on steranes
and terpanes in crude oils".
Above that temperature the steranes
included in the calcitc would decay.
An abundance of C27 steranes
relative to C29 counterparts, indicates the likelihood of an algal based origin with the analysis of the aromatic fraction indicating a very high level of thermal maturity (>1% RO).Based on the analysis to date, it is likely that the source of the oil is shallow marine carbonates of the Cambrian Ouldburra Formation and not from within the host Permian Mount Toondina sands.